The present invention relates generally to a method and apparatus for controlling the position of a web of material along a web processing path. More particularly, the present invention relates to a web positioning device for use with a gap drying device.
Web coating machines (i.e., web processing lines) are known in the art. The coating of a web requires first applying a coating to the web of material. In many coating applications, there follows a requirement to dry the coating on the web. With a typical web coating machine, the web travels along a path through the machine, to a coating station, then through one or more dryers and eventually to a winder. Any number of rollers, idlers, drive, brake and steering mechanisms may be located along the web coating machine. Additional processing equipment, for example an ultra violet (U.V.) station, may be further included as desired for particular web processing applications.
The coating station applies a coating to one face of the web. The “coated” face refers generally to a portion of the web of material which is sensitive to contact and contamination, though portions of the coated face may have been dried on the web. Coatings can be adhesives that are sensitive to any contact. Moreover, coatings are often applied with a liquid component (e.g., a solvent) that must be evaporated or otherwise removed before the coating is processed to a desired finished state.
Gap dryers, such as those described in U.S. Pat. Nos. 5,694,701 and 5,581,905, are known in the art for drying a coated web of material without the need for applied convection to evaporate and collect liquid coating materials. Gap dryers may be included in web coating machines. Gap dryers typically include a lower platen and an upper platen (also known as upper and lower plates) spaced from the lower platen by a relatively small gap. The web of material passes through a small gap between the upper and lower platens as the web travels through the coating machine. Passing the coated web between the upper and lower platens results in condensate forming on a surface of the upper platen. The upper platen forms a condensing structure for collecting condensate that has been evaporated from the web of material and for directing that condensate to a desired location. The upper platen can be chilled to facilitate the condensing process. In addition, the lower platen can be heated to further evaporation of the liquid from the web of material.
An air floatation oven, such as a convection dryer, may also be provided downstream of the gap dryer, for further drying of a coated web. The coated web generally passes through the gap dryer before passing through the air floatation oven, in order to avoid damage to the undried coating material caused by air movements in the air floatation zone.
As the web travels through the coating machine, inadvertent web upsets may occur. Upsets include any event that disrupts normal travel of the web through the coating machine, and include events that disrupt the longitudinal tension of the web. Upsets occur most often at the coating station and in the dryer. Such upsets lead to costly losses of time and materials. In particular, upsets can damage the sensitive coated face of the web. Upsets in a coating machine having a gap dryer can damage the coated web when the coated face of the web contacts the upper condensing platen of the gap dryer. Contact with the upper condensing platen can cause transfer of condensate from the upper condensing platen to the web, which can cause significant damage to the coated web, as well as raising safety and hygiene concerns. Contact of the web with the upper condensing platen can further cause contamination of the upper condensing platen, and contamination of capillary grooves of the upper platen with coating material is detrimental to both gap dryer functioning and machine operation.
Upsets also include web breaks, which are events that sever or tear a portion of the web. A change in the tension of the web, often a reduction in the tension, can lead to the web upset problems discussed above. In addition, web breaks often cause portions of the web to fall or pull through the web coating machine due to gravity. In that instance, the web may contact a ground surface, potentially contaminating the web and spreading undesired material to undesired areas, such as to other components of the coating machine and to the ground surface.
A web coating machine is generally characterized as including a number of tension zones. While the web may be generally secured at ends of each tension zone in the event of a web break, such tension zones may extend along a significant length of the web which can still pull through the web coating machine, and cause the types of difficulties described above.
In order to continue processing and coating a web when there is a break in the web through the coating machine, workers must splice severed portions of the web and then re-thread the spliced web through the coating machine. Splicing and re-threading the web through the coating machine, in particular re-threading the web through the air floatation oven, is difficult and time consuming. In addition, workers may re-thread portions of the web that have become contaminated, potentially spreading contamination to sensitive areas of the web coating machine. Because coating machine down time due to web upsets reduces the production output, it is important to limit the detrimental effects of inadvertent web upsets in order to maximize productivity and cost-effectiveness. Also, because precision coating processes have relatively narrow tolerances, web upsets can generate undesired waste.
Web breaks are most common at or near the following areas: the coating station, the air floatation oven, the unwinder, the winder, and at other processing equipment (e.g., the U.V. station). Also, web breaks are common at portions of the web where a splice has already been made. Splices are sometimes performed imperfectly, which can cause the splice to come undone and effectively cause a web break. Splices made with adhesives often come undone as the spliced portion of the web passes through the air floatation oven, due to elevated temperature.
Known mechanical and electrostatic web clamps, such as those disclosed in U.S. Pat. No. 4,462,528, can be used in conjunction with the web coating machine to hold the web in a static position. Holding the web in a static position prevents the web from pulling through the machine during inadvertent web breaks, and limits damage and disruption caused by web upsets. However, mechanical and electrostatic web clamps present a number of problems.
Mechanical web clamps contact both faces of the web, holding the web in a static position by frictional contact. Contact with a coated or wet face of the web causes damage to such a coated face, thereby generating waste product. In addition, contact with the coated material can contaminate the web clamp, generally necessitating cleaning of the mechanical web clamp after activation. Moreover, conventional mechanical web clamps can exhibit slow response times, reducing effectiveness of the mechanical clamp in preventing damage to the web from upsets when the tension of the web changes.
Electrostatic web clamps are limited in their usefulness. Electrostatic web clamps may be used only with insulative web materials, and not conductive web materials. Moreover, electrostatic clamps cannot be used in volatile and explosive material conditions, when the coating, the web, or other involved materials are volatile and/or explosive. In addition, electrical classification concerns are raised with the use of electrostatic web clamps, meaning electrostatic web clamps are typically limited to use in general purpose areas, absent significant additional costs. Moreover, electrostatic web clamps utilize face side brushes in close proximity to the coated surface of the web. Web flutter and contamination concerns are present due to the proximity of the brushes to sensitive coated areas of the web.
Also known are splicing machines that can hold a web or initiate a splicing procedure after a web break occurs. However, those splicing machines do not provide control over the positioning of a web upon a general web upset, nor do those splicing machines provide web positioning control with a web coating machine including a gap dryer.
Thus, an effective web positioning device is needed to provide control over the positioning of a web along a web processing path when the web advance is stopped.